In plastics injection moulding, molten plastics material forms layers as it fills the mould cavity. The first material entering the cavity forms the outer skin of the article being moulded and the rest of the material entering after this continues to form the core layers of the article. This phenomenon has been exploited by using two barrel injection moulding machines and injecting in sequence materials to form articles in which the skin is formed of a different material from the core, for example incorporating colour in the skin and then filling with cheaper uncoloured material, or by filling the skin with a plastics material containing fillers which would otherwise give an unsatisfactory surface finish.
In JP-A-5084733 there is disclosed an arrangement in which two separate materials are injected sequentially by two separate injectors into an accumulator, and then the sequence is expelled from the accumulator by a piston into the mould cavity to produce moulded articles with layers of the different materials. It will be seen that in this arrangement the materials are only formed into a sequence after being separately melted and plasticised in separate injectors. The necessary apparatus is costly and complex.
A disadvantage of such techniques is that the machinery required is costly and complicated.